A system for optical data transmission comprises essentially an optical transmitter with a power supply means for a light source device, which may be a light emitting diode (LED), an optical fiber which may be armoured to withstand high pressures and tensile stresses, and an optical receiver for detecting light pulses. The temperature characteristic of an LED produces a displacement of the emitted wavelength at increasing temperature. For a standard LED of 1300 .ANG. wavelength at ambient temperature, the wavelength is displaced to longer wavelengths at increasing temperatures, and at the same time the amplitude (or intensity) of the emitted radiation for the same level of input electrical power reduces with increasing temperature.
In addition to the problem caused by the reduced intensity of emitted light at high temperatures, water in the fiber environment may cause OH ions to be absorbed into the fiber, and these produce characteristic absorption peaks at 1240 .ANG. and 1390 .ANG. . The "water" absorption in the fiber will invariably increase with time, particularly in harsh operating conditions (i.e., high temperatures and pressures). The above-mentioned facts have not made it possible heretofore to reliably transmit light at temperatures of 200.degree. C. over long distances under water. The present invention seeks to make such optical transmissions possible.